The plant cell wall (PCW) is the most abundant biological resource available for the production of foods, fuels and materials, and plays a number of vital roles in plant growth and physiology. PCWs are comprised primarily of cellulose, lignin and a heterogenous group of polysaccharides known as hemicelluloses, along with varying amounts of pectins and proteins, depending on species and cell-type. Hemicelluloses are synthesized in the Golgi bodies of the plant cell, where glycosyltransferases (GTs) create polysaccharide chains via the transfer of monosaccharides from activated sugar donors. Here I have investigated the ability of GTs to function as recombinantly expressed enzymes in vitro for the controlled synthesis of polysaccharides with defined structures. I discuss within this volume the ability of an algal xylan backbone synthase to form unsubstituted xylan polymers which, at high degrees of polymerization, self-assemble into xylan microcrystals with distinct morphologies. In addition, I have investigated the effects of branching on the generation and morphology of these crystals, as well as their modification, degradation and potential applications. I also discuss the ability of a recombinant xylosyltransferase involved in xyloglucan biosynthesis to utilize non-native acceptor substrates in vitro. Finally, I also describe analytical techniques for the detection and charachterization of the methyl-ethers which decorate some sidechains of PCW polymers. These results provide greater understanding of PCW synthesis in vivo, enable new methods of PCW analysis, and facilitate the development of new in vitro methods for the biocatalytic synthesis of novel materials with defined properties that impart additional value to the next generation of bio-based materials and products.